The business of producing dynamic random access memory (DRAM) devices is a very competitive, high-volume business. Process efficiency and manufacturability, as well as product quality, reliability, and performance are the key factors that determine the economic success of such a venture.
Each cell within a DRAM device, an individually-addressable location for storing a single bit of digital data, is comprised of two main components: a field-effect access transistor and a capacitor. As component density in memory chips has increased, it has been necessary to at least maintain cell capacitance as cell size shrinks. Each new generation of DRAM devices generally has an integration level that is four times that of the generation which it replaced. Such a quadrupling of device number per chip is normally accompanied by a decrease in device geometries. All DRAM memories of 4-megabit and greater density utilize cell designs having three-dimensional capacitors. Although both trench and stacked capacitor designs have proven serviceable at the 4-megabit level, most manufacturers now seem to favor the stacked capacitor design for its manufacturability and somewhat higher yield.
Most current-generation dynamic random access memories (DRAMs) utilize CMOS technology. Although the term "CMOS" is an acronym for (C)omplementary (M)etal (O)xide (S)emiconductor, the term CMOS is now more loosely applied to any integrated circuit in which both N-channel and P-channel field-effect transistors are used in a complementary fashion. Although CMOS integrated circuit devices are often referred to as "semiconductor" devices, such devices are fabricated from various materials which are either electrically conductive, electrically nonconductive or electrically semiconductive. Silicon, the most commonly used semiconductor material can be made conductive by doping it (introducing an impurity into the silicon crystal structure) with either an element such as boron which has one less valence electron than silicon, or with an element such as phosphorus or arsenic which have one more valence electron than silicon. In the case of boron doping, electron "holes" become the charge carriers and the doped silicon is referred to as positive or P-type silicon. In the case of phosphorus or arsenic doping, the additional electrons become the charge carriers and the doped silicon is referred to as negative or N-type silicon. If a mixture of dopants having opposite conductivity types is used, counter doping will result, and the conductivity type of the most abundant impurity will prevail. Silicon is used either in single-crystal or polycrystalline form. Polycrystalline silicon is referred to hereinafter as "polysilicon" or simply as "poly". Although polysilicon has largely replaced metal for the MOS device gates, the relatively low conductivity of that material (even when heavily doped) has led many semiconductor manufacturers to create a layer of refractory metal silicide on transistor gates in order to decrease sheet resistance and, thereby, increase device speed. In conventional DRAM processes, two additional poly layers are used for the lower and upper cell capacitor plates.
CMOS manufacturing processes generally begin with a lightly-doped P-type or N-type silicon substrate, or lightly-doped epitaxial silicon on a heavily doped substrate. Although P-type silicon is usually chosen as the starting material, selection of N-type silicon as the starting material changes the process very little, the primary difference being that for a given step, dopant types are reversed.
Triple polysilicon layer manufacturing processes for contemporary stacked-capacitor dynamic random access memories require some fourteen to eighteen masking steps. As device geometries shrink, each photolithographic step becomes more costly. In light of the costs associated with masking operations, manufacturing processes which permit a significant reduction in the number of masking operations are greatly preferred.
In 1982, Japanese patent number 57-17164 was issued to Masahide Ogawa. This patent teaches the fabrication of a CMOS integrated circuit by processing N-channel and P-channel devices separately. As with conventional CMOS processes, a single polysilicon layer is used to form both N-channel and P-channel gates. However, N-channel devices are formed first, with unetched polysilicon left in the future P-channel regions until N-channel processing is complete. The mask used to subsequently pattern the P-channel devices is also used to blanket and protect the already-formed N-channel devices. This process is herein referred to as the split-polysilicon CMOS process. The split-polysilicon CMOS process, though largely ignored by semiconductor manufacturers in the U.S. and abroad., has been used extensively by Micron Technology, Inc. of Boise, Idaho as a means to reduce the number of masking steps and, hence, the cost of manufacturing dynamic random access memories.
U.S. Pat. No. 5,134,085 discloses a split-polysilicon CMOS DRAM process which incorporates conventional stacked capacitors. Although conventional stacked capacitors are adequate for 4-megabit and 16-megabit densities, more complex capacitors are required for the 64-megabit density level and beyond. Cylindrically-shaped cell capacitors (commonly referred to as container-type capacitors) have gained favor in 64-megabit DRAM designs because they are relatively easy to manufacture, and also because capacitance may be increased simply by increasing stack height. However, high topography which results generally requires the processing of P-channel devices early in the manufacturing process (i.e., before the container-type capacitors are fabricated). The problem with this approach is that boron trifluoride, the dopant normally used to dope P-channel source/drain regions, diffuses relatively rapidly under elevated temperature conditions. If the doping occurs early in the process, the thermal budget remaining in the process will cause the P-channel dopant to diffuse into the channel regions where it will produce short-channel effects. The most serious of those effects is device leakage when a high signal is applied to the gate.
What is needed, therefore, is a split-polysilicon CMOS DRAM process which incorporates container capacitors in an efficient process flow having a low number of masking steps, and which permits P-channel processing late in the process sequence.